Blender Git Loki

BGE showstopper bug fix:
- Reset hit object pointer at end of frame of touch sensor to avoid returning invalid pointer to getHitObject().
- Clear all references in KX_TouchSensor::m_colliders when the sensor is disabled to avoid loose references.
- Test GetSGNode() systematically for all KX_GameObject functions that can be called from python in case a python controller keeps a reference in GameLogic (bad practice anyway).